Project Summary While the clinical relevance of proteinuria, and especially albuminuria, has been well documented the quantitative mechanistic contribution or role of different contributing components to albuminuria remains an area of considerable excitement . In particular, the role of proximal tubules in albumin reabsorption and reclamation is now known to be an important determinant of the urinary barrier to albuminuria under physiologic and pathologic conditions. Therefore, the present application proposes to dissect apart and quantify the contributions of the known proximal tubule receptors cubilin/megalin and the fetal neonatal immunoglobulin receptor (FcRn) for albumin. To accomplish this we will quantify the interactions of cubilin and FcRn with albumin utilizing a stepwise and synergistic combination of biochemical solution binding assays, cell culture uptake, trafficking and receptor knock out studies in human proximal tubule cells and in vivo kidney studies using Munich Wistar Fromter rats with surface Glomeruli and 2-photon dynamic imaging. Biochemical, structural, functional and mechanistic observations will be interrelated to advance our present understanding of this clinically important phenomenon. Our Overall Hypothesis is that by understanding how albumin interacts with and is affected by these two receptors we will then understand how proximal tubule cells play fundamental, interactive and inducible roles to try and maintain the physiological state and minimize albuminuria. Our ultimate goal is to eventually develop a clinical approach that will allow quantification of the origin of albuminuria as either a proximal tubule or glomerular primary defect or a combination of both. This will allow for more specific therapeutic targets and agents to be identified. To directly evaluate this hypothesis we have developed the necessary techniques, approaches and cell and animal models to dissect, quantify and understand the process of proximal tubule metabolism of albumin.